High-pressure area
A high-pressure area, high, or anticyclone, is an area near the surface of a planet where the atmospheric pressure is greater than the pressure in the surrounding regions. Highs are middle-scale meteorological features that result from interplays between the relatively larger-scale dynamics of an entire planet's atmospheric circulation.
The strongest high-pressure areas result from masses of cold air which spread out from polar regions into cool neighboring regions. These highs weaken once they extend out over warmer bodies of water.
Weaker—but more frequently occurring—are high-pressure areas caused by
Within high-pressure areas, winds flow from where the pressure is highest, at the center of the area, toward the periphery where the pressure is lower. However, if the planet is rotating, the straight direction of the air flow from the center to the periphery is bent by the
On English-language weather maps, high-pressure centers are identified by the letter H. Weather maps in other languages may use different letters or symbols.
Wind circulation in the northern and southern hemispheres
The direction of wind flow around an atmospheric high-pressure area and a low-pressure area, as seen from above, depends on the hemisphere. High-pressure systems rotate clockwise in the northern Hemisphere; low-pressure systems rotate clockwise in the southern hemisphere.
High pressure systems may be either warm or cold types, the former originating in the subtropics and the latter at high latitudes, the time of year dictating which type is more dominant. Humidity and temperature of the high pressure system will depend on its source of origin. Warm high pressure systems from the horse latitudes (see below) create typical summer heat waves while cold high pressure systems bring freezing spells in winter and cooler, lower humidity in summer. If a high sits over the same area for several days it will take on the characteristics of that terrain. Cold high pressure systems in the Northern Hemisphere originate from Siberia, interior Canada, or the north Atlantic or Pacific, the latter two types trailing behind cyclonic systems. In the Southern Hemisphere, which is mostly water, these originate principally from the southern oceans.
The latitudes of 30N and 30S have semi-permanent high pressure around them known as the subtropical ridge, although their size and exact location varies with the seasons. On the West Coast of the United States, the subtropical ridge expands in spring and brings the region's characteristic rainless summer weather. As it shrinks in fall, the West Coast is subject to cold fronts from the Pacific which bring rain during the cool months. On the East Coast, it brings warm, humid air in late spring and throughout summer. In fall, as the subtropical ridge retreats, cold air from Canada takes over. In Europe, the effect is similar as the subtropical ridge brings the Mediterranean hot, dry summer weather and cool, wet winters. Europe north of the Pyrenees is at a higher latitude so the effect of the ridge is somewhat less significant and this region is mainly characterized by a cooler maritime climate. However, a particularly hot summer such as 2003 or 2019 which sees the subtropical ridge expand more than usual can bring heat waves as far north as Scandinavia—conversely, while Europe had record-breaking summer heat in 2003 due to a particularly strong subtropical ridge, its counterpart in North America was unusually weak and temperatures across the continent that spring and summer were wet and well below normal.[2]
In the Southern Hemisphere the result is similar. Australia and the southern cone of South America get hot, dry summer weather from the subtropical ridge and cooler wetter winter weather as cold fronts from the southern oceans take over.[3]
Winter sees the dominance of cold highs from the sub-Arctic. In Western Europe and the West Coast of North America, these originate in the Gulf of Alaska or the Greenland/Iceland area and move south to southeast. Since they are principally masses of ocean air, they will bring cool, damp conditions with widespread fog. In East Asia and interior North America, these air masses come from Siberia or Canada and bring very cold, dry air in their wake.
The scientific terms in English used to describe the weather systems generated by highs and lows were introduced in the mid-19th century, mostly by the British. The scientific theories which explain the general phenomena originated about two centuries earlier.
The term cyclone was coined by Henry Piddington of the British East India Company to describe the devastating storm of December 1789 in Coringa, India.[4] A cyclone forms around a low-pressure area. Anticyclone, the term for the kind of weather around a high-pressure area, was coined in 1877 by Francis Galton[5] to indicate an area whose winds revolved in the opposite direction of a cyclone. In British English, the opposite direction of clockwise is referred to as anticlockwise, making the label anticyclones a logical extension.
A simple rule is that for high-pressure areas, where generally air flows from the center outward, the
Formation
High-pressure areas form due to downward motion through the troposphere, the atmospheric layer where weather occurs. Preferred areas within a synoptic flow pattern in higher levels of the troposphere are beneath the western side of troughs.
On weather maps, these areas show converging winds (
High-pressure systems are alternatively referred to as anticyclones. On English-language weather maps, high-pressure centers are identified by the letter H in English,[8] within the isobar with the highest pressure value. On constant pressure upper level charts, it is located within the highest height line contour.[9]